298 HISTOLOGY OF THE MORBID PROCESSES. 



divided into zones, in which the direct damage is successively less 

 marked. In the first zone necrosis will have taken place ; in the 

 tissues that are more remote, degenerative changes will be occa- 

 sioned ; and still farther away from the seat of injury the tissues 

 will show a vital reaction to the stimulation or irritation they have 

 received, which will reveal itself in a growth, eventually leading to 

 a repair or patching of the defect in the tissues occasioned by the 

 damage. 



1. The Bloodvessels and the Circulation. The vessels most seri- 

 ously damaged, together with the blood they contained, will have 

 been completely destroyed ; in those less affected the circulation 

 will have been arrested and the blood coagulated. But beyond the 

 zones in which the function of the circulation has been abolished the 

 first marked effect is an increase in the volume and rapidity of the 

 current of blood. This increased flow of blood to the part is 

 attributed to the action of the injury upon the vaso-motor system 

 of nerves, causing a relaxation of the walls of the arteries' supply- 

 ing the part which has been damaged. A similar increase in circu- 

 lation follows slighter stimulation of the skin, as, e. g., rubbing, so 

 that this determination of blood to the part as the result of vaso- 

 motor disturbance is comparable with entirely normal hypersemias ; 

 but it is greater in degree when the irritation of the parts is great 

 enough to cause damage. 



After an interval the velocity of the circulation in the part which 

 is becoming inflamed is reduced, without any diminution in the 

 calibre of the vessels, and the slackening of the current may pass 

 into complete stasis. This is probably due to two causes : first, to 

 the extension of the vaso-motor disturbance beyond the area of the 

 injured part, so that collateral branches of the main arteries are 

 dilated ; this would diminish the pressure of blood going to the 

 inflamed part. Second, to alterations in the walls of the smaller 

 vessels in the inflamed part, especially the capillaries and small veins. 

 These become more pervious, probably as the result of the damage 

 they have sustained in common with the other tissues, allowing a 

 greater amount of fluid to pass through them than when they were 

 in the normal condition. This comparatively rapid extraction of 

 its watery constituent increases the viscosity of the blood, and that 

 increased viscosity, together with the changes in the walls of the 

 vessels, increases the friction between the two, impeding the cir- 

 culation. 



